This invention pertains to processing hydrocarbon feedstocks containing precipitable impurities which deposit out during preheating, and particularly to processing raw shale oil containing such precipitable materials to produce catalytically refined liquid fuels.
Attempts to hydrotreat and/or hydrocrack raw shale oil in conventional fixed-bed catalytic reactor operators, usually using multiple beds with quench steps between the beds, have been plagued by operating problems of fouling preheaters, plugging the beds, or both. Such fouling of flow passages and/or catalyst beds is evidently caused by precipitation of inorganic constituents of inorganic/organic complexes contained in the oil, and which decompose at temperatures below the desired hydrogenation reaction temperature. These precipitable inorganic materials contain arsenic and iron compounds and ash which cannot be readily filtered out of the feed stream at near ambient conditions. Shale oil contains small quantities of metals, such as about 60 ppm iron and 10 ppm arsenic, as weakly bonded chemical complexes. These compounds evidently decompose at about 500.degree.-600.degree. F. and precipitate the metal, which deposits on solid surfaces, thereby plugging heater tubes and fixed catalyst beds.
It has been proposed to provide a low temperature guard bed containing particulate solids to remove the metals, and then heat the effluent to the hydrogenation reaction conditions of 700.degree. F. or more required in a second catalytic reactor. However, the deposits occurring in such guard traps cause high pressure drops and even plugging, so that they are inconvenient and expensive to use. Thus, a better solution has been sought for avoiding or preventing such metal compound deposits and fouling problems in processing raw shale oil, so as to permit continous catalytic treating such oils to produce upgraded fuel products.
The multi-stage catalytic processing of heavy petroleum crude oils and residuum is known. For example, U.S. Pat. No. 3,705,849 to Alpert discloses a process for desulfurization of petroleum residuum feedstocks using ebullated catalytic bed hydrogenation reactors in series to reduce hydrogen consumption and increase catalyst life. U.S. Pat. No. 3,773,653 to Nongbri and U.S. Pat. No. 3,788,973 to Wolk dislose similar multistage catalytic conversion processes for petroleum residuum. Also, U.S. Pat. No. 3,887,455 to Hamner discloses a process for hydrotreatment of heavy crudes and residua using ebullated catalytic beds or fixed-bed reactors in series, using catalyst having smaller pore size in the second reactor.
U.S. Pat. No. 4,046,670 to Seguchi discloses a process for thermal cracking heavy petroleum oil in a tubular type heating furnace, and wherein an inorganic substance containing iron oxide is added to the feed as an anti-clogging agent. U.S. Pat. No. 4,181,596 to Jensen discloses treating shale oil retort effluent to lower pour point and reduce contaiminants, such as soluble arsenic and iron, by cooling the effluent and maintaining the liquid phase in a critical temperature range of 600.degree.-800.degree. F. for 1-120 minutes. Also, U.S. Pat. No. 4,158,622 to Schwarzenbek discloses a two-stage hydrogenation process for hydrocarbons such as shale oil containing particulate fines, utilizing an ebullating bed catalytic reactor from which a vapor portion is passed to a stationary bed reactor for further hydrotreatment.
Despite the prior activity, a need still exists for improvements in processing raw shale oil which contains precipitatable inorganic materials and compounds so as to avoid fouling of equipment passages and catalytic beds and provide improved operations. The present invention uses the hydrogenation exotherm in an ebullated bed reactor to eliminate the feed heater and any guard bed, and then finish hydrotreats the light material in a fixed bed catalytic reactor. Inorganic materials are deposited in the ebullated bed reactor on the catalyst, and the deposits are removed from the bed along with the used catalyst.